A double-hung sash window assembly typically includes a window frame and a pair of window sashes mounted for vertical reciprocal sliding movement, one relative to the other, on guide rails in the master frame jamb of the window assembly. Although window sashes traditionally have been made exclusively of wood, window sashes are often formed of extruded plastic or metal frame members which are joined at mitered corners, for instance, to form a generally rectangular frame in which glazing is mounted. An example of this type of window sash is described in U.S. Pat. No. 4,578,903, having the same assignee as the present invention.
Window sashes also are known which include a sash lock or fastener which, when the sash is closed, automatically, locks and holds the sash in the closed position. The fastener includes a rectangular casing having an opening at its front end or face plate, in which opening a tumbler or dog is mounted. The casing is secured in the front surface of a window stile. The tumbler is mounted within the casing by a guiding device so that the tumbler may be pivotally projected forward or longitudinally to overlie the lower sash, and be moved laterally when projected forward to engage the casing and lock itself in position. The tumbler is biased into the projected position by a coiled, safety pin type spring mounted within the casing. This is more fully discussed in U.S. Pat. No. 4,923,230.
It has been found that in applications where large forces are applied to prior art tumblers, such as disclosed in the '230 patent, the backwall of the casing can deform under the increased horizontal force against the backwall imposed by the tumbler, and allow the tumbler protrusion to slide along the back wall and over-rotate, permitting the windows to open.
The present invention is provided to solve this and other problems.